# Patent

## Title
Method of using a formable core block for a resin impregnation process

## Abstract

A use of a core block for an impregnation process as well as a composite structure comprising such a core block is described. The core block has a first surface and a second surface, and a number of first grooves is formed in the first surface of the core. Furthermore, a number of second grooves is formed in the second surface of the core. The first grooves have a first height (h1) and a bottom, and the first grooves and the second grooves are part of a resin distribution network formed in the core block. The distance (t) between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves. Additionally, the sum of the first height and the second height is larger than the thickness of the core block, and at least one of the first grooves in the first surface of the core block crosses at least one of the second grooves in the second surface of the core block.

## Background
This is a U.S. national phase Application filed under 35 U.S.C. 371 of PCT/DK2008/000243, filed 27 Jun., 2008 claiming priority benefit from DK Application No. (s) PA 2007 00945, filed 29 Jun. 2007 and PA 2007 00946, filed 29 Jun. 2007, the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD
The invention relates to a use of a core block for a resin impregnation process, such as vacuum assisted resin transfer moulding, and a method of forming a composite structure as well as a composite structure obtainable via the method.
BACKGROUND
Thus the invention relates to a method for producing fibre composite structures and composite structures obtained by means of VARTM (vacuum assisted resin transfer moulding), where liquid polymer, also called resin, is filled into a mould cavity, in which fibre material priorly has been inserted, as a vacuum is generated in the mould cavity hereby drawing in the polymer. The polymer can be thermoset plastic or thermoplast.
Vacuum infusion or VARTM is a process used for moulding fibre composite mouldings, where uniformly distributed fibres are layered in one of the mould parts, the fibres being rovings, i.e. bundles of fibre bands, bands of rovings or mats, which are either felt mats made of individual fibres or woven mats made of fibre rovings. The second mould part is often made of a resilient vacuum bag, and is subsequently placed on top of the fibre material. By generating a vacuum, typically 80 to 95% of the total vacuum, in the mould cavity between the inner side of the mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained herein. So-called distribution layers or distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases the polymer applied is polyester or epoxy, and the fibre reinforcement is most often based on glass fibres or carbon fibres.
During the process of filling the mould, a vacuum, said vacuum in this connection being understood as an underpressure or negative pressure, is generated via vacuum outlets in the mould cavity, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels the polymer disperses in all directions in the mould cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus it is important to position the inlet channels and vacuum channels optimally in order to obtain a complete filling of the mould cavity.
Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Thus dry spots are areas where the fibre material is not impregnated, and where there can be air pockets, which are difficult or impossible to remove by controlling the vacuum pressure and possibly an overpressure at the inlet side. In connection with vacuum infusion, employing a rigid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by for example puncturing the bag in the respective location and by drawing out air for example by means of a syringe needle. Liquid polymer can optionally be injected in the respective location, and this can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mould parts, staff have to stand on the vacuum bag. This is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus in a local weakening of the structure, which can cause for instance buckling effects.
Patent literature discloses examples of the use of a semi-permeable membrane, which increases the area, in which the vacuum is active, and thus reduces the above problems. In this connection the term semi-permeable membrane means a membrane, which is permeable to gasses but impermeable to liquid polymer. Thus if a semi-permeable membrane is placed across the fibre insertion, air pockets can be removed more easily or prevented entirely.
Typically the composite structures comprise a core material covered with a fibre reinforced material, such as one or more fibre reinforced polymer layers. The core material can be used as a spacer between such layers to form a sandwich structure and is typically made of a rigid, lightweight material in order to reduce the weight of the composite structure. In order to ensure an efficient distribution of the liquid resin during the impregnation process, the core material may be provided with a resin distribution network, for instance by providing channels or grooves in the surface of the core material. EP 0 831 987 and EP 0 1304 211 disclose a method and a composite structure, respectively, in which such a core material is used.
As for instance blades for wind turbines have become bigger and bigger in the course of time, and may now be more than 60 meters long, the impregnation time in connection with manufacturing such blades has increased, as more fibre material has to be impregnated with polymer. Furthermore the infusion process has become more complicated, as the impregnation of large shell members, such as blades, requires control of the flow fronts to avoid dry spots, said control may e.g. include a time-related control of inlet channels and vacuum channels. This increases the time required for drawing in or injecting polymer. As a result the polymer has to stay liquid for a longer time, normally also resulting in an increase in the curing time.
WO 2007/098769 describes one method for optimising the process of producing composite structure.
It is an object of the invention is to obtain a new method for producing composite structures, such as sandwich structures, by for instance VARTM processes, and which overcomes or ameliorates at least one of the disadvantages of the prior art or which provides a useful alternative. It is also an object to provide a new and improved composite structure obtainable via the new method.
DISCLOSURE OF THE INVENTION
This is according to a first aspect of the invention obtained by a use of a core block for a resin impregnation process, such as vacuum assisted resin transfer moulding, wherein the core block having a first surface and a second surface and having a thickness, a number of first grooves being formed in the first surface of the core block, the first grooves having a first height and a bottom, and a number of second grooves being formed in the second surface of the core and having a second height, and the first grooves and the second grooves being part of a resin distribution network formed in the core block, wherein the sum of the first height and the second height is larger than the thickness of the core block, and wherein at least one of the first grooves in the first surface of the core block crosses at least one of the second grooves in the second surface of the core block, and wherein the distance between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves.
According to a second aspect, the invention provides a method of forming a composite structure comprising the following steps: a) providing at least one core block having a first surface and a second surface and having a thickness, a number of first grooves being formed in the first surface of the core, the first grooves having a first height and a bottom, and a number of second grooves being formed in the second surface of the core and having a second height, the first grooves and the second grooves being part of a resin distribution network formed in the core block, b) covering at least a part of the core block with a fibre material, c) placing the covered core block in a forming structure comprising a mould part and a vacuum bag, d) sealing the vacuum bag against the mould part to form an interior part of the forming structure, e) connecting a source of uncured fluid resin so as to feed uncured resin to the resin distribution network, f) connecting the interior of the forming structure to at least one vacuum outlet, g) forcing the uncured resin from the source of uncured resin through the resin distribution network and to the vacuum outlet in order to fill the interior of the forming structure and to impregnate the fibre material, and h) curing the resin in order to form a composite structure, wherein the sum of the first height and the second height is larger than the thickness of the core block, and wherein at least one of the first grooves in the first surface of the core block crosses at least one of the second grooves in the second surface of the core block, and wherein the distance between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves.
According to a third aspect, the invention provides a composite structure comprising: at least one core block having a first surface and a second surface and having a thickness, a number of first grooves being formed in the first surface of the core, the first grooves having a first height and a bottom, and a number of second grooves being formed in the second surface of the core and having a second height, the first grooves and the second grooves being part of a resin distribution network formed in the core block, a fibre material covering at least a part of the core block, and a cured resin impregnating the fibre material and the resin distribution network, wherein the sum of the first height and the second height is larger than the thickness of the core block, and wherein at least one of the first grooves in the first surface of the core block crosses at least one of the second grooves in the second surface of the core block, and wherein the distance between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves, when not impregnated by cured resin.
Hereby, the core blocks with resin distribution network are formed as a single integrated block, which alleviates the need of placing separate small quadrilateral blocks on a foil or a net, thereby facilitating easier handling of the blocks, when arranging these in the mould part. Furthermore, since the first grooves in the first surface of the core blocks are nearly cut all the way through the core block, the core block becomes highly flexible, which means that they can be placed and formed according to the curvature of the mould part. This is highly desirable when producing composite structures having a curved exterior surface, such as for instance wind turbine blades. Thus, the first grooves in the first surface form bending lines for the core blocks. Therefore, the term “flexible” has the meaning that the core block can follow the curvature of the mould part during impregnation, for instance when vacuum is applied to the mould or form part. Further, the resin can be supplied from one side of the core blocks to the other side of the core blocks without the need to provide bore holes in the core blocks, since at least one of the first grooves are connected to at least one of the second grooves. The grooves in the core blocks can for instance be cut in the blocks by use of a CNC cutter or milling machine. Additionally the cured resin filling the resin distribution network helps to prevent delamination of the composite structure or the resulting sandwich structure.
When the composite structure is a shell part, such as a shell half part for a wind turbine blade, steps a)-c) are typically performed by first placing one or more fibre layers in the mould part and defining the outer surface of the finished composite structure, then placing the core blocks on top of the fibre layers and finally placing one or more fibre layers on top of the core block and defining the inner surface of shell part.
The invention also provides broader aspects, where the core blocks are formable, i.e. a core block not necessarily having second grooves provided in the second surface of the core block.
Thus, according to a first broader aspect of the invention, the invention provides the use of a core block for a resin impregnation process, such as vacuum assisted resin transfer moulding, wherein the core block has a first surface and a second surface, a number of first grooves being formed in the first surface of the core, the first grooves having a first height and a bottom, the first grooves being part of a resin distribution network formed in the core block, and wherein the distance between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves.
According to a second broader aspect, the invention provides a method of forming a composite structure comprising the following steps: a) providing at least one core block having a first surface and a second surface, a number of first grooves being formed in the first surface of the core, the first grooves having a first height and a bottom, the first grooves being part of a resin distribution network formed in the core block, covering at least a part of the core block with a fibre material, placing the covered core block in a forming structure comprising a mould part and a vacuum bag, sealing the vacuum bag against the mould part to form an interior part of the forming structure, connecting a source of uncured fluid resin so as to feed uncured resin to the resin distribution network, connecting the interior of the forming structure to at least one vacuum outlet, forcing the uncured resin from the source of uncured resin through the resin distribution network and to the vacuum outlet in order to fill the interior of the forming structure and to impregnate the fibre material, and h) curing the resin in order to form a composite structure, wherein the distance between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves.
According to a third broader aspect, the invention provides a composite structure comprising: at least one core block having a first surface and a second surface, a number of first grooves being formed in the first surface of the core, the first grooves having a first height and a bottom, the first grooves being part of a resin distribution network formed in the core block, a fibre material covering at least a part of the core block, and a cured resin impregnating the fibre material and the resin distribution network, wherein the distance between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves, when not impregnated by cured resin.
In the following, various advantageous embodiments are described. The embodiments are applicable both to the first, second and third aspects of the invention as well as the broader aspects of the invention.
In a first embodiment according to the invention, the first height is at least 75% of the thickness of the core block. Alternatively, the first height is at least 80%, 85%, 90%, or 95% of the thickness of the core block.
In a second embodiment according to the invention, the distance between the bottom of the first grooves and the second surface of the core block is between 0.3 and 5 mm, alternatively between 0.4 and 3 mm, or alternatively between 0.5 and 2.5 mm. These sizes have been found to provide an appropriate bending motion for the core blocks. The thickness of the individual core blocks may vary according to the desired core thickness. According to one embodiment, the thickness of the blocks varies between 20 and 35 mm.
According to one embodiment, the core block comprises a foam material, such as foamed polymer. The foamed polymer can for instance be chosen from the group of polystyrene, polyurethane, polyethylene, polypropylene, polyvinyl chloride (PVC), cellulose acetate, polystyrene, or nylon. Alternatively, the core blocks comprise balsa wood and/or concrete.
The resin is preferably chosen from the group of vinyl ester, polyester, epoxy, or phenolic, acrylic of bismaleimide resin.
The core blocks may have a varying stiffness in order for the flexibility of the blocks to be varying as well. If the core blocks for instance are made of PVC, the stiffness can be varied by the amount of foamed polymer per cubic meter. Typically this amount varies between 80 and 100 kg/m3.
According to another embodiment, the first grooves in the first surface of the core blocks are substantially parallel.
In one embodiment according to the invention, the first grooves in the first surface of the core block are extending substantially transverse to the second grooves in the second surface of the core block. That is, the first grooves in the second surface of the core block can also be parallel. This provides a particular simple solution, where the first grooves cross the second grooves at many points.
According to one embodiment, the core block additionally comprises a number of additional grooves in the first surface, the additional grooves having a height smaller than that of said first grooves. The additional grooves are preferably arranged transverse to the first grooves.
According to an advantageous embodiment, the width of the grooves are between 0.1 and 5 mm, alternatively between 0.2 and 3.5 mm, or alternatively between 0.3 and 2.5 mm. The grooves of a core block can have a mutual centre spacing of between 10 and 50 mm, alternatively between 12 and 40 mm, and alternatively between 15 and 35 mm.
According to another advantageous embodiment, the height of the part of the first grooves in the first surface crossing the first grooves of the second surface is between between 0.25 and 5 mm, alternatively between 0.3 and 3.5 mm, or alternatively between 0.35 and 2.5 mm. That is, the sum of the first height and the second height minus the thickness of the core blocks should lie in the above intervals.
In general, the grooves should be sized and arranged to allow resin to impregnate the fibre material efficiently before curing. For instance, it is important that the cross section of the part of the grooves in the first surface crossing the grooves in the second surface are appropriately dimensioned in order not to supply the resin from the upper side to the lower side of the blocks too quickly or too slowly.
According to an alternative embodiment, the second grooves have a second bottom, and the distance between the bottom of the second grooves and the first surface of the core block is of such a size that the core block is flexible along the second grooves. Thereby, a simple way of providing a core block, which is flexible in both a longitudinal and a transverse direction, is provided. The second height may for instance be at least 75% of the thickness of the core block. Alternatively, the second height is at least 80%, 85%, 90%, or 95% of the thickness of the core block. The distance between the bottom of the second grooves and the first surface of the core block may for instance be between 0.3 and 5 mm, alternatively between 0.4 and 3 mm, or alternatively between 0.5 and 2.5 mm.
As described in WO EP 0 831 987 and EP 0 1304 211, fibre material may be provided between the individual core blocks. Furthermore, the plurality of individual core blocks may be covered with a further fibre material, either before or during placement in the mould.
According to one embodiment, a plurality of core blocks are arranged adjacent each other. Thereby, individual blocks can be used to form a larger core part and thereby a larger composite structure. In order to obtain an efficient distribution of liquid resin, at least a number of the grooves of a given core block can be aligned with grooves of an adjacent core block.
The sidewalls of the first grooves and/or the second grooves may be diverging, e.g. shaped as a u- or a v-groove. Thereby, an additional degree of flexibility is provided.
According to another embodiment, the composite structure to be produced is an oblong structure, such as a wind turbine half shell, and has a longitudinal direction and a transverse direction, and wherein the first grooves in the first surface of the core blocks are arranged substantially in the longitudinal direction. Therefore, the mould part is also oblong and the first grooves are aligned in the longitudinal direction of the mould part.
According to yet another embodiment, a fibre insertion is additionally placed in the mould part, the fibre insertion comprising a plurality of fibre layers and forming a main laminate, which in the finished composite structure constitutes a longitudinally extending reinforcement section. The finished composite structure, such as a wind turbine shell part, may comprise additional reinforcement sections, such as one or two main laminates extending in the longitudinal direction of the shell part and/or side or edge reinforcements.
According to one embodiment, a gelcoat is applied to the mould part prior to step a). Thereby, the gelcoat forms the hard, outer surface of the finished composite structure. Furthermore, the inner surface of the mould part can be covered with a waxy substance in order to prevent the composite structure to adhere to the mould. Alternatively, a peel ply may be used.
According to another embodiment, the resin is supplied to the resin distribution network via supply tubes arranged across the composite structure to be produced, i.e. the supply tubes are arranged on top of the core blocks or more precisely on top of the fibre layers defining the inner side of the finished half shell. The supply tubes are preferably arranged in the longitudinal direction of the mould part. The tubes can be formed as so-called omega profiles with an opening facing the composite structure to be produced.
Alternatively, one or more of the core blocks may comprise one connection fitting, and a supply tube is inserted in the fitting.
According to yet another embodiment, the vacuum outlet comprises one or more vacuum channels, which are connected to a vacuum source, the vacuum channels preferably being arranged in the longitudinal direction of the mould part. The vacuum outlet may comprise a number of centrally arranged vacuum channels and/or vacuum channels provided at the sides of the mould part.
According to an advantageous embodiment, the vacuum channels are provided with a semi-permeable membrane being substantially permeable to gasses and substantially impermeable to resin. The vacuum channels can for instance be formed as a profile member with a slit opening, the semi-permeable membrane being provided at the opening. The vacuum channels and/or the supply tubes can be part of the vacuum bag.
According to one embodiment, a tear-off layer is additionally placed on top the covered core blocks. This layer can for instance be a peel ply or a holey foil. Hereby, the resin supply tubes, vacuum channels, semi-permeable membranes, vacuum bag etc. can be removed easily after curing has finished by tearing off the tear-off layer, on top of which all these elements are placed. The tear-off layer can for instance be in the form of a perforated film or a fibre layer.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in detail below with reference to an embodiment shown in the drawings, in which
FIG. 1 is a schematic view of a wind turbine shell part, seen from above,
FIG. 2 a schematic cross section through a mould part with fibre layers and core material placed in the mould part,
FIG. 3 a schematic view of a first core block according to the invention, seen in perspective,
FIG. 4 a cut along line A-A in FIG. 3,
FIG. 5 the core block shown in FIG. 4 being slightly flexed,
FIG. 6 a cut along line B-B in FIG. 3,
FIG. 7 a first composite structure according to the invention,
FIG. 8 a second composite structure according to the invention,
FIG. 9 a schematic view of a second core block according to the invention, seen in perspective,
FIG. 10 a schematic cross section through an arrangement for carrying out a method according to the invention, and
FIGS. 11a-h are schematic cross sections showing how liquid polymer is distributed in the fibre insertion by the method according to the invention.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a wind turbine blade shell part 1 seen from above. The shell part comprises a first fibre insertion 2, also called the main laminate, which extends along at least a part of the longitudinal direction of the shell part 1. The main laminate 2 functions as a reinforcement section for the finished wind turbine blade.
The main laminate 2 extends from the blade root 7 to the tip end 8. The main laminate is here depicted as having a uniform width; however, typically the main laminate has a varying width and becomes narrower towards the tip end 8 of the shell part 1. For large wind turbine blade, such as the LM61.5 blade, the shell parts may additionally comprise a second fibre insertion 3, also called the small main laminate. Additionally, the shell part 1 may comprise reinforcement sections, such as fibre insertions, at a leading edge 4 and a trailing edge 5 of the wind turbine blade shell part 1, respectively.
FIG. 2 shows a cross-sectional view through a mould part 110 having a mould cavity, in which a number of fibre layers, core parts and reinforcement sections are placed, these parts being included in a finished wind turbine shell part. FIG. 2 is only used to show the relative placement of all these parts in a wind turbine shell part and an example of how to arrange resin inlet channels and vacuum outlets. The blade shell part comprises one or more lower fibre layers 114 impregnated with resin and optionally coated with a gel-coat, which define the exterior surface of the shell part, and one or more upper fibre layers 113 impregnated with resin, and which define the interior surface of the shell part. The upper fibre layer(s) 113 and lower fibre layer(s) 114 are separated by a fibre insertion or main laminate 102 comprising a plurality of fibre layers impregnated with resin, a first core part 111 and a second core part 112, as well as a first fibre reinforcement 115 at a trailing edge 105 of the shell part and a second fibre reinforcement 116 at a leading edge 106 of the shell part.
The arrangement for the VARTM process comprises a number of vacuum outlets, where a first vacuum outlet 131 and a second vacuum outlet 133 are placed above the main laminate 102 as well as a first additional vacuum outlet 160 and a second additional vacuum outlet 165 arranged at rims or flanges of the rigid mould part 110, i.e. at the leading edge 104 and trailing edge 105 of the finished blade shell part, respectively.
The resin is supplied via a number of resin inlet channels 125, 126 arranged above the main laminate as well as a number of first additional resin channels 170 above the first core part 111 and a number of second additional resin channels 175 arranged above the second core part 112.
The shell part, is in FIG. 2 depicted as having a uniform thickness. However, typically the shell part is thickest around the main laminate 102 and becomes thinner towards the leading edge 104 and the trailing edge 105. The shell part is here depicted as having only one main laminate, but it may comprise a second or small main laminate as well, in which case the shell part also comprises a third core part.
FIG. 3 shows a core block 30 according to the invention, which can be used for the core part of a wind turbine shell part as shown in FIG. 1 or 2 or for a sandwich structure in general. FIGS. 4 and 6 show cross sections of the core block along lines A-A and B-B, respectively.
The core block 30 comprises a first surface 35 and a second surface 36. A number of substantially parallel first grooves 31 are formed in the first surface 35. The core blocks 30 are typically arranged such that the first grooves extend in the longitudinal direction of the composite structure or sandwich structure of which the core blocks 30 are part. A number of second grooves 32 are formed in the second surface 36 of the core block. The second grooves 32 are arranged substantially transverse to the first grooves 31.
Furthermore, a number of additional grooves 33 are formed in the first surface 35 of the core block. These are also arranged transverse to the first grooves 31. The additional grooves 33 may as shown in FIG. 3-6 be arranged in the same planes as the second grooves 32. However, this is not necessary, and the grooves may be mutually displaced in the longitudinal direction of the core block 30.
The first grooves 31 have a first depth h1, and the second grooves 32 have a second depth h2, while the total thickness of the core block 30 is given by h. The distance from the bottom of the first grooves 31 to the second surface 36 of the core block 30 is denoted t (which is equal to h minus h1). The distance between adjacent grooves is denoted d, and the widths of the individual grooves are denoted w. Normally, the individual grooves have the same width and spacing, however, these dimensions may vary in order to optimise the distribution of liquid resin.
The core block 30 is made of a rigid, lightweight material, such as balsa wood or foamed polymer, for instance foamed polyvinyl chloride (PVC).
The first grooves 31 extend nearly all the way through the block, such that the distance between the bottom of the first grooves 31 and the second surface 36 is of such a thickness t that the core block is flexible or bendable about axes extending in the longitudinal direction along the first grooves 31. FIG. 5 shows an example of the core block 30 being bent about these axes. Thereby, the core blocks can be formed according to the mould part, in which they are placed, and be formed according to the surface of the mould cavity, when a vacuum is applied to the mould cavity.
Further, the second grooves 32 may extend nearly all the way through the block, such that the distance between the bottom of the second grooves 32 and the first surface 36 is of such a thickness that the core block is flexible or bendable about axes extending in the transverse direction along the second grooves 32.
FIGS. 7 and 8 show examples of using such core blocks for sandwich structures. FIG. 7 shows a first sandwich structure comprising a first core block 40 and a second core block 50, which are covered by a first fibre layer 45. The sandwich structure is depicted with the sides of the first core block 40 and the second core block 50 abutting each other. However, the core blocks may also be arranged with a small mutual spacing for distributing resin. Preferably, the individual core blocks are arranged such that grooves of one core block are aligned with grooves of the other core block in order to obtain an efficient distribution of resin during the impregnation process.
The grooves of the core blocks 40, 50 and the first fibre layer 45 are filled with cured resin, and the resin in the grooves helps to prevent delamination of the composite sandwich structure. The sandwich structure may of course also comprise only one core block or comprise three or more core blocks, and the blocks can have varying surface thicknesses in order to obtain the desired shape for the composite structure.
FIG. 8 shows a second sandwich structure. In this embodiment, each core block is covered by a separate fibre layer. Thus, the second sandwich structure comprises a first core block 60 covered by a first fibre layer 65 and a second core block 70 covered by a second fibre layer 75. These two blocks are covered by an additionally fibre layer 85.
When making large composite structures, such as the wind turbine shell parts shown in FIGS. 1 and 2, the core parts often comprise core blocks of different materials in order to optimise the weight and strength of the composite structure. The core parts may for instance comprise core blocks of balsa wood and foamed PVC, respectively.
Since, balsa wood and foamed PVC have different rigidities and densities, the combination of such core blocks may give rise to boundaries, where the strength of the sandwich structure is different on either side. Such discontinuities may give rise to local weaknesses due to stress lines or areas during use of the composite structure, especially if the structure is a wind turbine blade. Such areas with discontinuous strength increase the probability of buckling effects of the composite structure. Therefore, the core blocks which form boundaries to core blocks of a different material or strength are typically tapered in order to obtain a gradual transition from one type of core material to another.
Such a core block 30′ with a tapered region 37′ is depicted in FIG. 9. This core block 30′ also comprises a number of substantially parallel first grooves 31′ formed in a first surface 35′ of the core block 30′. As with the core block depicted in FIG. 3, the tapered core block 30′ also comprises a number of transverse arranged second grooves 32′ formed in a second surface of the core block 30′. The core block 30′ additionally comprises a number of transverse arranged additional grooves 33′ formed in the first surface 35′ of the core block 30′. The tapered region 37′ is tapered with an angle α.
From FIG. 9 it is seen that the height (h1) of the first grooves 31′ and the height of the core block (h) may vary for instance in the transverse direction of the core block 30, whereas the distance (t) between the bottom of the first grooves 31′ and the second surface 36′ of the core block 30′ is substantially constant. Of course small fabrication variations may occur.
The grooves of the core block can for instance be made by cutting by use of a CNC cutter or milling machine. The grooves are in FIG. 3-9 depicted as having square sides; however, the may also be slightly tapered in order to be more bendable or flexible in the other direction of the one depicted in FIG. 5.
FIG. 10 shows a cross section through a mould for manufacturing a blade shell half for a blade of a wind turbine by means of vacuum infusion. The figure shows a solid mould part 210 with a top side matching the exterior side of the finished blade shell half. For illustrative reasons the top side of said mould part 210 is shown as being plane, but it is typically curved to the desired form of the blade shell half. First, a waxy substance is applied to the mould cavity of the mould part 210 in order to prevent the shell half to adhere to the surface. Subsequently, a so-called gel coat is placed on the inner side of the mould part 210, said gel coat later on forming the outside of the finished blade shell half. Above the gel coat is placed a fibre material consisting of a plurality of lower, preferably thin, fibre layers 214, above which a lower distribution layer 229 is placed. A fibre insertion 202 is placed on top of the lower distribution layer 229 and the fibre layers 214. A first core part 211 functioning as a filling material or sandwich material, such as balsa or PVC foam, is placed adjacent a first side of the fibre insertion 202, and a second core part 212 is placed adjacent a second side of the fibre insertion 202.
The fibre insertion 202 functions as a reinforcement section, also called a main laminate, of the blade and extends in the longitudinal direction of the blade shell part. The fibre insertion 202 comprises a plurality of fibre layers. The fibre insertion 202 comprises a first zone 221 at the first side of the fibre insertion 202 and a second zone 222 at the second side of the fibre insertion 202. The first zone 221 and the second zone 222 are separated in a transverse direction of the blade shell part by an intermediate zone 220.
An upper distribution medium 223 in form of a plate-like element provided with grooves 224 for distributing liquid polymer is placed above the intermediate zone 220 of the fibre insertion 202. The upper distribution medium 23 is also called a top hat in popular speak.
A fibre material in form of a plurality of upper, preferably thin, fibre layers 213 is placed above the top hat 223, fibre insertion 202, and the first and second core part 211, 212. The resin may be supplied via a device for providing polymer, which is laid into the mould part 210 on top of the upper fibre layers 213, so that it extends in the longitudinal direction of the mould part 210. The device for providing polymer thus includes a tear-off layer or a peel ply 240, for instance in form of a holey foil, a first vacuum outlet or vacuum channel 231 provided with a first semi-permeable membrane 232, a second vacuum outlet or vacuum channel 233 provided with a second semi-permeable membrane 234 as well as a number of resin inlet channels 225-228.
The top hat 223 is placed so that it overlaps the intermediate zone 220 of the main laminate 202, and the first vacuum channel 231 and the second vacuum channel 233 are placed so that they overlap the first 221 and the second zone 222 of the main laminate 202, respectively.
The lower distribution layer 229 can for instance be made from a porous core material, e.g. balsa or foamed polymer, provided with channels formed as recesses in the surface and extending in the plane of the distribution layer, often perpendicular to the longitudinal direction of the blade. The channels can, however, also extend at any other possible angle compared to the longitudinal direction of the blade. Alternatively, the lower distribution layer 229 is a net or a fibre mat with high permeability to liquid resin. The lower distribution layer 229 is in FIG. 10 depicted in an embodiment overlapping the entire main laminate 202. However, it is also possible to provide two different distribution layers, which overlap the first zone and the second zone, respectively, and which extend in below the intermediate zone 220 of the main laminate 202. The main importance is that the lower distribution layer or layers are arranged such that they are able to make contact with flow fronts of liquid resin propagating down through the intermediate zone 220 and through the first core part 211 and second core part 212, so that the flow fronts are drawn in below the first zone 221 and the second zone 222 of the main laminate 202.
At the top an airtight vacuum bag 250 is placed, together with the mould part 210 forming a mould cavity or equivalently a forming part with an interior. Furthermore, vacuum channels, e.g. in the form of perforated vacuum tubes, can be placed at the flanges, i.e. the rims, of the mould. Additionally, a number of inlet channels (not shown) can be provided above the first core part and the second core part, equivalently to the embodiment shown in FIG. 2.
The core parts 211, 212 may comprise any of the core blocks shown in FIGS. 3-9 or a combination of such blocks and core blocks known in the art. The core blocks are as previously described typically provided with channels extending in the plane of the core blocks perpendicular to and in the longitudinal direction of the blade shell half. The channels can also be formed by placing balsa blocks on a permeable cloth or net. These channels ensure that liquid polymer can propagate quickly in the plane of the core parts. The core parts need not, however, be balsa or foamed PVC, but can be of any other material with good flow qualities. This could for instance be a loosely woven mat of glass fibre or a similar net structure with high permeability and large capacity.
Typically, the core parts 211, 212 comprise a combination of core blocks of balsa wood and foamed PVC in order to optimise the weight and strength of the blade shell part.
During the process of filling the mould the first and the second vacuum channels 231, 233 and possibly other applied vacuum channels communicate with a vacuum source, and the inlet channels 225-228 communicate with a polymer source with liquid polymer. The vacuum in the vacuum channels creates a vacuum in the entire mould cavity between the mould part 210 and the vacuum bag 250, hereby drawing resin through the inlet channels 225-228 into the mould cavity as the polymer propagates through and impregnates the fibre materials 213, 214, 202 as well as the core parts 211, 212. When curing is completed, the vacuum bag 250 and the peel ply 240 are removed together with the vacuum channels 231, 232 and resin inlet channels 225-228.
For the resin inlet channels 225-228, Ω-shaped profile members or perforated tubes known per se can be used. The vacuum channels can also be formed as Ω-shaped profile members or perforated tubes. If perforated vacuum tubes are applied, they may be reinforced by a helical rigid member extending into the tube and preventing it from collapsing due to the vacuum.
However, the use of resin inlet channels under the vacuum bag 250 may cause problems, since the vacuum bag will press the inlet channels against the fibre layer or main laminate 202, which can leave impressions in these fibre layers or main laminate 202. These impressions may give rise to buckling effects during the subsequent use of the wind turbine blade, which could mean costly repairs of the blade or even replacement of the blade.
Such problems are reduced or completely remedied by the top hat 223. The top hat 223 balances or equalises this pressure to the entire lower surface of the top hat 223 and thus prevents the formation of impressions in the main laminate 202. The top hat 223 is tapered towards the sides of the top hat 223 in order to obtain a gradual transition between different materials of the finished shell part; this again being in order to prevent the formation of boundaries with a discontinuous stiffness or strength, thus minimising the probability of the afore-mentioned buckling effects. In order to keep the top hat 223 as thin as possible, it is usually made of balsa wood. However, it could also be made of foamed polymer. The grooves of the top hat 223 may have any of the configurations shown in FIGS. 3-9. However, it can also be provided with first grooves formed in the upper surface and extending in the longitudinal direction and with second grooves formed in the lower surface and extending in the transverse direction, the upper and lower surface being connected by a number of through-going holes. Alternatively, the grooves can be formed equivalent to the grooves of the core blocks shown in FIGS. 3-9.
It is not shown in FIG. 10, but the leading edge and the trailing edge of the blade shell part typically have rim reinforcements of fibre material as shown in FIG. 2.
During the process of filling the mould, liquid resin flows from the inlet channels 225-228 into the mould cavity due to the vacuum in the mould cavity defined by the mould part 210 and the vacuum bag 250. Due to the vacuum in the entire mould cavity, the liquid polymer is both drawn towards the leading edge and trailing edge of the blade shell part and into the fibre insertion 202.
FIGS. 11a-h show how the fibre material of the main laminate 202 is impregnated with liquid resin during the process of filling the mould. Prior to the start of the process of filling the mould, air is evacuated in the mould cavity by means of the first and the second vacuum channel 231, 232 and possibly other vacuum channels, which e.g. can be placed at the leading edge and/or trailing of the blade shell part. The evacuation of the air creates a vacuum which is to be understood as a negative pressure in the mould cavity.
FIG. 11b diagrammatically shows the start of the process of filling the mould, where the inlet channels 225-228 are filled with liquid polymer, indicated by the grey tinted areas. At the start of the process of filling the mould, liquid polymer is directed to the top hat 223, the grooves 224 of which are quickly filled with resin as shown in FIG. 11c. Hereafter the polymer slowly spreads down through the main laminate 202 as shown in FIG. 11d, as the liquid polymer creates a wide flow front moving down through the intermediate zone 220. At the same time, some of the polymer is drawn into the first zone 221 and the second zone 222 towards the first semi-permeable membrane 232 and the second semi-permeable membrane 234, respectively.
At the beginning of the filling process, the resin is supplied only from the inlet channels 225-228 above the top hat 223 for a predetermined amount of time, for instance 30 minutes. Subsequently, resin is additionally supplied from additional resin inlet channels (not shown) arranged over the first core part 211 and second core part 212, respectively, thus creating flow fronts propagating through the resin distribution networks of these core parts. These flow fronts are shown at both sides of FIG. 11d. The additional resin inlet channels can be arranged equivalently to the embodiment shown in FIG. 2.
When the three flow fronts reach the lower distribution layer 229 as shown in FIG. 11e, the resin quickly spreads herein and thus in below the first zone 221 and second zone 222 of the main laminate 202. Subsequently, as shown in FIG. 11f, the flow fronts move upwards through the first zone 221 and the second zone 222 and finally, as shown in FIG. 11g, form desired v-shapes ensuring that no air pockets arise. At the end, the entire main laminate 202 is impregnated as shown in FIG. 11h, whereafter the curing process is started.
The top hat 223 ensures that liquid polymer propagates in the main laminate 202 with a relatively wide flow front on the way down through the intermediate zone 220, and the lower distribution layer 229 ensures a wide flow front on the way up through the first zone 221 and the second zone 222. As the width of the main laminate is often many times the size of the thickness of the main laminate, it is ensured that the flow front of polymer is to move the shortest distance possible, whereby the impregnation time can be reduced. Since the resin only needs to stay liquid for a shorter time, the subsequent curing time can also be reduced. Thereby the total time for manufacturing e.g. a blade for a wind turbine can be reduced substantially, which means that the total production time can be reduced to fewer working-shifts than with previously used methods. Thereby, the overall production cost can be lowered substantially.
To ensure that the flow front of polymer only moves slowly transversely in the main laminate 202 towards the first zone 221 and the second zone 222, the material of the main laminate 202 can be structured in such a way that it has a higher permeability across the fibre layers than in the plane of the fibre layers.
To ensure that air pockets do not arise during the process of impregnation it is furthermore important that the thickness of the main laminate 202 and the width of the intermediate zone 220 are mutually dimensioned appropriately.
The inlet channels 225-228 can be dimensioned so that a large amount of liquid polymer can flow through them. At the end of the process of filling the mould the polymer source can be closed prior to closing the vacuum source, whereby the inlet channels 7, 8 can be emptied of liquid polymer more easily. This reduces the polymer wastage.
The vacuum channels 231, 233 can be formed by the semi-permeable membrane 232, 234 and a cloth, wherein spacer material can be placed in the form of a three-dimensional net or the like in order to ensure that the membrane 232, 234 and the cloth do not collapse during the process of filling the mould, during which a vacuum is established in the vacuum channel 231, 233.
EXAMPLE
According to a given prototype of a LM61.5 wind turbine blade, the width of the main laminate differs in the longitudinal direction of the blade. The main laminate is approximately 850 mm wide and has a thickness of approx. 42 mm at the widest and thickest part, respectively. The width of the top hat also varies in the longitudinal direction of the blade and is approximately 360-370 mm at the widest part and 150 mm at the slimmest part.
The core blocks have the following dimensions: w=1-1.2 mm, d=30 mm, h2=2.5+/−0.5 mm, α=14 degrees, and t=1 mm. The core blocks have varying thickness h, and therefore the height of the first grooves is also varying. The overlap between the first grooves and the second grooves are equal to h2 minus t and is thereby 1.5-2.0 mm.
The examples have been described according to preferred embodiments. However, the invention is not limited to these embodiments. For example, the width and height of the individual grooves as well as the distance between grooves should be dimensioned in order to obtain the optimum trade-off between an efficient distribution of the liquid resin during the impregnation process and a subsequent weight distribution of the cured resin. It is also clear, that the overlap between first grooves and second grooves should be optimised in order to supply liquid resin from the first surface to the second surface with a desired speed. Thus, this overlap should neither be too small nor too large.
LIST OF REFERENCE NUMERALS


    1 wind turbine blade shell part
    2, 102, 202 main laminate/fibre insertion
    3 small main laminate/small fibre insertion
    4, 104 leading edge
    5, 105 trailing edge
    7 blade root
    8 tip end
    30 core block
    31 first grooves
    32 second grooves
    33 additional grooves
    35 first surface
    36 second surface
    37′ tapered region
    40 first core block
    45 first fibre layer
    50 second core block
    60 first core block
    65 first fibre layer
    70 second core block
    75 second fibre layer
    85 additional fibre layer
    110, 210 mould part
    111, 211 first core part
    112, 212 second core part
    113, 213 upper fibre layer(s)
    114, 214 lower fibre layer(s)
    115 first fibre reinforcement
    116 second fibre reinforcement
    125-126 resin inlet channels
    150, 250 vacuum bag
    160 first additional vacuum outlet
    165 second additional vacuum outlet
    170 first additional resin inlet channel(s)
    175 second additional resin inlet channel(s)
    220 intermediate zone
    221 first zone
    222 second zone
    223 plate-like distribution medium/top hat
    224 grooves
    225-228 resin inlet channels
    229 lower distribution layer
    231 first vacuum outlet
    232 first semi-permeable membrane
    233 second vacuum outlet
    234 second semi-permeable membrane
    240 tear-off layer/holey foil/peel ply

## Claims

1. A method of forming a composite structure comprising the following steps:
a) providing at least one core block having a first surface and a second surface and having a thickness (h), a number of first grooves formed in the first surface of the core, the first grooves having a first height (h1) and a bottom, and a number of second grooves formed in the second surface of the core block and having a second height (h2), the first grooves and the second grooves forming part of a resin distribution network in the core block, and the core block bendable about axes extending in the longitudinal direction along the first grooves,
b) covering at least a part of the core block with a fiber material,
c) placing the covered core block in a forming structure comprising a mold part and a vacuum bag,
d) sealing the vacuum bag against the mold part to form an interior part of the forming structure,
e) connecting a source of uncured fluid resin to feed uncured resin to the resin distribution network,
f) connecting the interior of the forming structure to at least one vacuum outlet,
g) forcing the uncured resin from the source of uncured resin through the resin distribution network and to the vacuum outlet in order to fill the interior of the forming structure and to impregnate the fiber material, and
h) curing the resin in order to form a composite structure, wherein the sum of the first height and the second height (h1+h2) is larger than the thickness (h) of the core block, and wherein at least one of the first grooves in the first surface of the core block crosses at least one of the second grooves in the second surface of the core block, wherein the distance (t) between the bottom of the first grooves and the second surface of the core block is of such a size that the core block is flexible along the first grooves, and wherein the core block is formed as a single integrated block.

2. A method according to claim 1, wherein the composite structure to be produced is an oblong structure and has a longitudinal direction and a transverse direction, and wherein the first grooves in the first surface of the core blocks are arranged substantially in the longitudinal direction.

3. A method according to claim 1, wherein additionally a fiber insertion is placed in the mold part, the fiber insertion comprising a plurality of fiber layers and forming a main laminate, which in the finished composite structure constitutes a longitudinally extending reinforcement section.